1. Field of the Invention
This invention is in the field of fluorine chemistry and more particularly in the field of direct fluorination.
2. Description of the Prior Art
Although polymers have been known and manufactured for many years, it has only been recently that significant efforts have been initiated to take advantage of functional groups on polymer surfaces. Such functional groups could provide new uses for the base polymers due to the nature of the functional group, or the functional group could provide a reactive site on the polymer surface so that a variety of chemical compounds could be reacted with the polymer.
One of the earlier applications for functionalized polymers involved the development of peptide syntheses based on chloromethylated polystyrene. See G. R. Stark, "Biochemical Aspects of Reactions on Solid Supports", Academic Press, New York (1971). These procedures avoided repetitious purifications in the syntheses of complicated peptides, and such techniques have become so successful that these "Merrifield resins" are now widely used and are commercially available. Chloromethylated polystyrene has also been used in more conventional organic synthesis work such as the promotion of intramolecular reactions without high dilution apparatus and the separation of polymer bound triphenylphosphine oxide after a Wittig reaction. See J. I. Crowley and H. Rapaport, J. Amer. Chem. Soc., 92, 6363 (1970); M. A Draus and A. Patchornik, Israel J. Chem., 9, 269 (1971); S. V. McKinley and J. W. Rakshys, Chem. Comm. 134 (1972); and, F. Camps, J. Castells, J. Font and F. Vela, Tetrahedron Lett., 1715 (1971).
Polystyrene has also been functionalized and subsequently used to form heterogeneous catalysts by reacting homogeneous catalysts with the functional groups. See R. H. Grubbs, C. Gibbons, L. C. Kroll, W. D. Bonds and C. H. Brubaker, J. Amer. Chem. Soc., 95, 2373 (1973); and, R. H. Grubbs, L. C. Kroll and E. M. Sweet, J. Macromol. Sci. Chem., A7, 1047 (1973). Heterogeneous catalysts have advantages over their homogeneous counterparts because they can be prepared in a more reactive form since dimerization reactions that occur in solution cannot occur with the bound species and because they are more easily recoverable.
In addition to the effort directed to the functionalization of polystyrene, some effort has been made to functionalize polyethylene. It has been recognized that polyethylene functionalized with carboxylic acid groups, for example, would be particularly useful because of the chemical and mechanical properties of polyethylene and because of the ease with which carboxylic acid units can be converted to esters, amides, ketones, etc. Several different oxidation procedures have been reported in the literature as successfully forming carboxylic acid groups on polyethylene surfaces. See B. G. Aristov, I. Yu. Babkin, F. K. Borisova, A. V. Kiselev and A. Ya. Korolev, Izv. Acad, Nauk, SSSR, Otd. Khim., 6, 1017 (1963); Akad, Nauk, SSSR, Bulletin, 927 (1963); F. H. Ancker and F. L. Baier, U.S. Pat. No. 3,556,955 (1971); and, R. L. Augustine, Ed., "Oxidation, Vol. I", Marcel Dekker, New York, 1969, p6.
Although most of the effort to functionalize polymers to date has been directed towards simply adding acid or other reactive groups to polystyrene and polyethylene, it would be desirable to be able to add functional groups to the class of polymers known as fluoropolymers. Fluoropolymers are known to exhibit outstanding high temperature properties and are also unusually chemically inert. Because of these properties, they are used in applications where severe environmental factors are encountered.
One attempt to functionalize and fluorinate polymers is the method described by Manly in U.S. Pat. No. 3,865,616. This method involves contacting polyolefins or certain polyesters with a mixture of oxygen and fluorine to produce oxyfluorinated polymers containing reactive carboxyl groups. Plasminokinase enzymes, which activate plasminogins to plasmin, are then reacted with the carboxyl groups to form non-thrombogenic surfaces. Suitable polyolefins are disclosed to be polyethylene, polypropylene or ethylene-propylene copolymers. Suitable polyesters are disclosed to be those formed by reacting a dibasic carboxylic acid with a difunctional alcohol to form a condensation polyester having the repeating unit --[CO(R)COO(R.sub.1)O]-- wherein R and R.sub.1 are independently chosen from linear or cyclic hydrocarbons.
Functionalization of fluoropolymers has heretofore been very limited, however, probably due to a number of factors. Fluoropolymers are extremely inert so that functionalization of the fully fluorinated polymer is unlikely to succeed. Alternatively, polymerization of fluorinated monomers containing functional groups is difficult for steric reasons, i.e., the bulkiness of the functional group interferes with polymerization.
Another problem has been the difficulty in carrying out the fluorinations themselves. Whereas many compositions can be directly chlorinated or brominated, it has been recognized that fluorine is dissimilar to these halogens in regard to direct halogenation. See McBee et al., U.S. Pat. No. 2,533,132 and U.S. Pat. No. 2,614,129. In fact, even though direct fluorination is a highly desirable process, prior attempts to use direct fluorination have often produced low to mediocre yields. Additionally, the yields are known to decrease as the molecular complexity of reactants becomes greater, thereby making direct fluorination of polymers an even more difficult matter. It is stated in one literature article, for example, that the yield of required fluorocarbon decreases as the molecular complexity of a hydrocarbon precurser increases, and it is difficult to fluorinate hydrocarbons above C.sub.10 without extensive decomposition occurring. See R. E. Banks, "Fluorocarbons and Their Derivatives", Oldbourne Press, London, p. 7 (1964).
Direct fluorination reactions involving elemental fluorine are also characterized by quick evolution of large quantities of heat, ignition and flaming which promote product decomposition, often with explosive violence. In fact, the inability to control direct fluorination reactions to produce high yields of the desired fluorinated reactant without concomitant fragmentation of the desired product has prevented direct fluorination from becoming a widely accepted method of fluorination. Because of these problems, a very diversified art has been developed to circumvent or obviate the use of fluorine gas by using inorganic, metallic fluorides, hydrogen fluoride, or electrolytic cells where no free fluorine is produced.
Direct fluorination of polymers containing pendant groups has even been more limited. In fact, it is suggested in the patent literature that treatment of polyfluoroalcohols with elementary fluorine results in destructive fragmentation of the carbon chain and loss of the functional group at the end of the chain. See Stallmann, U.S. Pat. No. 3,038,941.